ASP.NET Zero sends emails to users in some cases (like forgot password and email confirmation). Email template is defined in Emailing/EmailTemplates folder of .Core project (default.html). You can change default email template by editing this file.
Email sending is disabled in DEBUG mode. Because, development environment may not be configured properly to send emails. You can enable it if you want. It's enabled in RELEASE mode. Check YourProjectNameCoreModule class's PreInitialize method to change it if you like.
.NET Core Compatibility
Since .NET Core does not support smpt client, ASP.NET Zero uses MailKit to send emails.